It is known that crustaceans have the neurogenic heart whose myocardial cells contract under tight control of rhythmically active neurons in the cardiac ganglion. We have previously demonstrated that myocardium of Bathynomus doederleini was sensitive to glutamate. In pharmacological tests, glutamate, quisqualate and kainate developed tension of myocardial cells in a dose-dependent manner. The dose-response relationship showed an EC50 of about 10-4 M quisqualate, about 3*10-4 M glutamate, and about 5*10-4 M kainate. The order of potency was thus quisqualate>glutamate>kainate. We analyzed glutamate in the extracts of the cardiac ganglion and myocardium of the Bathynomus by means of a technique, high performance liquid chromatography (HPLC). The analysis revealed, respectively, 8741.2+/-184.2pmol/mg and 678.2+/-10.7 pmol/mg for glutamate contents. The present study may show that motor neurons in the cardiac ganglion are glutamatergic.It has long been repeated that acetylcholine accelerates the heart of decapod crustaceans. Yazawa and Kuwasawa (1992) recently suggested that acetylcholine is the primary candidate for a neurotransmitter of small neurons in the cardiac ganglion of the hermit crab, Aniculus aniculus. We performed a HPLC analysis (with electrochemical detection) of the cardiac ganglion of A.aniculus and Homarus americanus, and detected acetylcholine in the cardiac ganglia of both the animals. We analyzed acetylcholine content in the cardiac ganglia of H.americanus. It was 129+-3.2 fmol/mg wet weight. The present study support the previous postulation (Yazawa and Kuwasawa, 1992) . We conclude that the small neurons in the cardiac ganglion of decapod crustaceans may be cholinergic nerurons, since other neurons and neuronal processes in the ganglion are revealed to be dopaminergic and GABAergic (Yazawa and Kuwasawa, 1992,1994).